Generally, in preparing liquid crystal display (LCD) devices, liquid crystal cells including liquid crystals and polarizers are basically required and suitable adhesive layers or pressure-sensitive adhesive layers have to be used for binding them. In addition, for improving functions of LCD devices, a phase retardation plate, a compensation plate for wide view angle, a brightness enhancing film, and the like may be used, with additionally adhered to the polarizer.
The main structure forming an LCD device includes, generally, a uniformly aligned liquid crystal layer; a polarizer with a multi-layer structure, incorporated into a pressure-sensitive adhesive layer or an adhesive layer, based on a liquid crystal cell consisting of a transparent glass plate or a plastic sheet material containing a transparent electrode layer; a phase retardation plate; and an additional functional film layer and the like.
The polarizer has a structure including an iodine compound or a dichroic polarizing material aligned in a certain direction. To protect these polarizing elements, multi-layers are formed on both sides using a protective film such as triacetyl cellulose (TAC). In addition, the polarizer may additionally include a phase retardation film, a compensation film for wide view angle such as a liquid crystal type film, a brightness enhancing film, a reflective layer, or a smoke layer in a shape having a unidirectional molecular alignment.
The aforementioned films are made of materials having different molecular structures and compositions, and so have different physical properties. In particular, under high temperature or high temperature and humidity conditions, the dimensional stability according to shrinkage or expansion of materials having a unidirectional molecular alignment is insufficient.
As a result, if the polarizer is fixed by a pressure-sensitive adhesive, then stress remains by shrinkage or expansion of the polarizer under high temperature, or high temperature and humidity conditions. As a result, the pressure-sensitive adhesive layer is deformed and high polymers in the crosslinking structure are aligned in a specific direction due to the residual stress, whereby birefringence is developed. Under this alignment, a general alkyl acrylic pressure-sensitive adhesive shows a negative birefringence, and thus light leakage occurs due to the birefringence difference.
As a method of minimizing light leakage under the residual stresses, a birefringence caused by the pressure-sensitive adhesive under the residual stress may be minimized by adding (blending) materials having a positive birefringence under the residual stress to the final pressure-sensitive adhesive layer or copolymerizing acrylic monomers having a positive birefringence.
Korean Laid-Open Patent Publication No. 2003-0069461 discloses a technical idea of correcting a negative birefringence of an acrylic pressure-sensitive adhesive layer under the residual stress by incorporating a low-molecular weight material having a positive birefringence under the stress into 0.01-40 part by weight of the acrylic pressure-sensitive adhesive layer. However, in this case, the modulus of the pressure-sensitive adhesive is lowered due to the low-molecular weight materials incorporated into the pressure-sensitive adhesive layer. As a result, there is a problem in tailoring property in processing the polarizer and there are problems such as the moving of the low-molecular weight material into the interface for long-term storage and the possibility of phase separation with the acrylic pressure-sensitive adhesive.
A method of minimizing birefringence by copolymerizing a monomer having a negative birefringence and a monomer having a positive birefringence under the residual stress is disclosed in “Applied Optics” (1997). As a specific example, the degree of birefringence may be regulated under the given stress by copolymerizing an acrylic monomer (negative birefringence) having a side chain of an alkyl group and an acrylic monomer (positive birefringence) having a side chain of an aromatic group.
Japanese Patent Laid-Open Publication No. 2002-332468 discloses a method of improving plastic resistance of the pressure-sensitive adhesive layer by introducing an acrylic monomer containing an aromatic group in a side chain. In addition, U.S. Pat. No. 6,663,978, and Japanese Patent Laid-Open Publication Nos. 2002-173656 and 2003-013029 disclose a method of regulating a refractive index of a pressure-sensitive adhesive layer by introducing an acrylic monomer containing an aromatic group in a side chain. Further, Japanese Patent Laid-Open Publication No. 2005-053976 discloses a method of improving pressure-sensitive adhesion performance in even low polar films by introducing an acrylic monomer containing an aromatic group in a side chain. However, in those patents, there are not notified technical ideas of attempting optical compensation which regulates birefringence under the residual stress to improve the light leakage phenomenon, when acrylic pressure-sensitive adhesives for polarizing plates are prepared by introducing an acrylic monomer containing an aromatic group in a side chain.
Japanese Patent Laid-open Publication No. 1986-207101 discloses a method to prepare a pressure-sensitive adhesive by mixing an acrylic copolymer (A) having a crosslinking functional group, an acrylic copolymer (B) not having a crosslinking functional group, and a multi-functional crosslinking agent having at least two functional groups, wherein the weight ratio of A/B is in the range of 1/4˜4/1. Although this Publication discloses a technical feature that the shelf life of the pressure-sensitive adhesive can be extended by removing a free functional group through addition of a corresponding amount of multi-functional crosslinking agent to the amount of the crosslinking functional group during the crosslinking process, it does not disclose any condition on the molecular weight of an acrylic polymer used therein, nor a technical feature of the crosslinking structure. Particularly, the patent does not disclose a technical feature of the stress release property of a pressure-sensitive adhesive.
In a liquid crystal display method of a twisted nematic (TN) type or a super twisted nematic (STN) type, polarizers are generally attached onto both sides of a liquid crystal panel with their optical axes being crossed at 45° and 135°, for which light leakage caused by the residual stress is crucial.
However, in a liquid crystal display (LCD) device such as In-Plane Switching (IPS) type or Vertical Alignment (VA) type mainly used in large scale TVs, polarizers are attached onto both sides of a liquid crystal panel with their optical axes being crossed at 0° and 90°. For these LCD devices, light leakage caused by bending of a liquid crystal panel as well as light leakage caused by stress concentration in a heat or moist heat environment is known to be a crucial factor. The bending of the liquid crystal panel is caused by a difference in shrinkage of the polarizer (especially, the elongation axis direction) according to position under a heat or moist heat condition. That is, TN type or STN type LCD devices are structured such that the polarizers are attached with their optical axes being crossed at 45° and 135° and thus the elongation axis direction of the upper polarizer and the elongation axis direction of the lower polarizer are positioned symmetrically or in a balanced way, whereas LCD devices such as IPS type or VA type are structured such that the polarizers are attached with their optical axes being crossed at 0° and 90° and thus the elongation axis direction of the upper polarizer and the elongation axis direction of the lower polarizer are positioned asymmetrically. As a result, in the LCD devices such as IPS type or VA type, bending of the upper polarizer and bending of the lower polarizer are different from each other, for which the liquid crystal panel is bent in a direction that the degree of bending is large. Due to such bending of the liquid crystal panel, the liquid crystal panel is pressed by a top case used to fix the liquid crystal panel, resulting in light leakage caused by non-uniformity of liquid crystals.
Korean Patent Laid-Open Publication No. 10-2006-0060264 discloses an LCD device having a feature that an upper polarizer and a lower polarizer have pressure-sensitive adhesive layers, respectively, and the pressure-sensitive adhesive layer of the upper polarizer and the pressure-sensitive adhesive layer of the lower polarizer have different pressure-sensitive adhesive strengths according to the direction of bending of a liquid crystal panel, thereby improving the bending phenomenon. However, with this method, it is difficult to control the degree of bending of the upper polarizer and the lower polarizer according to an environment (heat or moist heat condition).
Japanese Patent Laid-Open Publication No. 2006-58718 discloses a pressure-sensitive adhesive for a polarizer having a storage modulus of 105 to 109 Pa and a glass transition temperature of −20° C. or higher at 0 to 50° C. after crosslinking to suppress light leakage of an LCD device of an IPS or MVA type, thereby achieving higher modulus and glass transition temperature than those of conventional pressure-sensitive adhesives. However, this patent does not mention improvement associated with bending of a liquid crystal panel of an IPS or MVA type and with the method suggested in this patent, it is difficult to solve the light leakage problem caused by bending of the liquid crystal panel.
Therefore, there is an urgent need for development of a new pressure-sensitive adhesive for a polarizer, which can solve the light leakage problem caused by bending of a liquid crystal panel as well as light leakage caused by residual stress, without changing main features of a polarizer product such as durability under a long-term use condition as well as a heat or moist heat condition, and a polarizer adopting the new pressure-sensitive adhesive.